If using a three-piece type combination oil ring provided with a top and bottom pair of side rails and a spacer expander arranged between the same, unburned carbon or carbon produced from the burnt products of the lubricating oil tend to deposit between the spacer expander and side rails.
If this advances, the deposits cause the side rails and spacer expander to freeze up, the side rails can no longer mold to the shape of the cylinder bore, and the predetermined performance as an oil ring can no longer be exhibited. As opposed to this, in Japanese Utility Model Publication (B2) No. 2-31559, at center parts having the substantially flat surfaces between peak parts and valley parts in the expander, holes of sizes sufficient for passage of the deposits are formed.